1244 THE STRUCTURE OF EVOLUTIONARY THEORY
red or our bones white for any directly adaptive reason rooted in the colors
themselves. But although transparency surely stands as a primary prerequisite, many
other enzymes and proteins share this necessary property, but have never been
recruited as lens crystallins—so more specific preconditions must be sought (Wistow,
1993; Piatigorsky, 1993a and b). Piatigorsky (1993a, p. 285) lists "high solubility in
water to achieve the high concentrations necessary to attain the appropriate refractive
index and thermodynamic stability, since loss of cell nuclei in the fiber cells prevents
turnover in this region of the lens."
Even more specifically, Wistow (1993, pp. 303-304) notes that all cellular lenses
require unusually elongated cells as building blocks—a common property or
potential, as Wistow argues, of the original utilities from which lens crystallins have
been exapted: "As the lens evolved, the necessary refractive power must have been
achieved by recruiting genes that are active under the prevailing conditions of cell
elongation and whose protein products fit the broad requirements of their new role.
Osmotic stress proteins, cytoskeleton chaperones and easily inducible detoxification
enzymes would have been good candidates. Such an origin could have engendered
underlying similarities in gene expression for groups of crystallins."
But the case of crystallins owes its emerging status as a "classic" of exaptation
largely to the strong evidence gathered for a range of structural prerequisites and
preconditions that can facilitate such functional shifts. Arnold (1994) has proposed a
set of subcategories for sources and styles of exaptation (see also Gould and Vrba,
1982), and I shall devote the final section of this chapter (pp. 1277-1294) to the
further development of such taxonomy and to exploring its implications for
macroevolutionary patterns and possibilities. The subject has assumed some urgency
in studies of molecular evolution because the crucially important mechanism of gene
duplication has frequently been overextended and interpreted as virtually the only
possible basis for exaptation—when a gene with an important function duplicates
(Ohno, 1970, for the classic statement), thereby "freeing" one copy for cooptation to a
different utility. Exaptation does occur by duplication in the evolution of some lens
crystallins, but other exapted crystallins are products of a single gene that continues
to make the critical enzyme of its presumably original function—a process that
Piatigorsky and Wistow (1989) called "gene sharing," and that Darwin explicitly
recognized in citing organs with two distinct functions as good candidates for quirky
functional shift (see p. 1223).
For example, the duck genome includes only a single gene to code for both the
exapted lens crystallin and the original enzyme in at least two cases: epsilon crystallin
(lactate dehydrogenase B) and tau crystallin (alpha-enolase). In some cases, the lens
crystallins even retain their enzymatic activity within the eye. The zeta crystallin of
several hystricomorph rodents is quinone oxido-reductase, and may protect the eye
against oxidation "or even filter UV radiation" (Wistow, 1993, p. 301). The amount
of epsilon crystallin in many birds, produced by the same single-copy gene that codes
for the enzyme lactate dehydrogenase B, also correlates well with exposure to light,
and may